A typical DC to DC converter includes a rectifier, usually a Schottky diode, to virtually eliminate current flow in the reverse direction. A Schottky diode provides a low forward voltage drop, normally around 0.3 volts. This low forward drop provides low power loss while current conducts through the diode.
Referring to FIG. 1, a circuit 100 depicts a typical example of a DC to DC converter. When the switch 106 is closed, current flows through the circuit loop including a DC power supply 101 (usually a battery source), an inductor 102 and the switch 106 so that the inductor 102 builds up and stores energy. When the switch is opened, the energy stored in the inductor 102 is converted to current and conducts through a Schottky diode 103 and stored as energy in a capacitor 104.
A switch 106 is toggled to convert an input DC voltage level to an output DC voltage level. Conventional converters use an MOS transistor as the switch 106. After the inductor 102 is discharged into the capacitor 104, the energy stored in the capacitor 104 drives the load 105. The rectifying effect of the Schottky diode prevents the energy stored in the capacitor 104 from flowing through the switch to ground or back through the inductor to the battery.
Unfortunately, the circuit has a parasitic energy loss due to the forward voltage drop across the Schottky diode 103. During the time that the diode 103 is conducting the power loss is approximately equal to the voltage drop times the forward current.
The circuit has three operating modes. In the first mode the inductor 102 is charged. In the second mode, the switch 106 is opened and the energy stored in the inductor 102 is discharged into the capacitor 104. In the third operating mode, the system enters a quiescent mode until another charging cycle is needed. Typical durations for the quiescent state to the active charging states will vary with load current and may be on the order of 1000:1 for light loads. For example, the charging and discharging activity may take place in 10 .mu.S while the circuit is quiescent for 10 mS.
To avoid the problems associated with the 0.3 volt drop across a typical Schottky diode, others have used a P-type MOSFET and a control circuit as the rectifier. The voltage drop across a typical P-type MOSFET is in the 10-100 mV range when conducting. However, an active control circuit is required to control such MOSFET rectifiers by switching them between the triode region and cut-off. Prior art control circuits necessarily draw current in order to effectuate the control of the MOSFET rectifier. Because of the relative duration of the quiescent mode, this current draw for the control circuit is significant. What is needed is an active rectifier in a DC to DC power converter having a control circuit that draws no current except when the rectifier conducts current.